What are the red flags when buying a used airgun?

Vague or evasive answers about modifications, 'I never shot it' combined with a worn finish, missing factory accessories, refusal to fill or pressurize a PCP for inspection, no recent target photos, scope mount marks suggesting it has been re-mounted many times, repaired or swapped serial markings, and a price that is too good to be true. PCPs that have sat empty for years often have leaky seals waiting to surface. Modifications can void warranties even on used guns sold within their warranty period.